I recommend you inspect some existing 3d models of rifles matching your standard (e.g. search "rifle" on sketchfab, check UVs). Unlike in your example, you will find that on several rifles the UVs of the stock consist of 2 islands with the seams running vertically along the gun. UV islands shape represent shape of the mesh…
Here is a quick dirty example of how to solve this issue. If you leave your stock as one piece you wont have this problem. However, if you need to put a seam in your butt stock to get better Texel density, I would put the seam in a spot that is easy to stamp out in Substance Painter. Add your material, then make a paint…
so how can i texture something like this to get the wood grains to look right, i have attached the stock in case someone had the time to fix the issue as above rearrange the mapping so seams are better managed/believable. Otherwise you can use rather old school render to texture technique's to perfectly match the textures…